1. Field of the Invention
The subject invention concerns an integrated circuit layout for LFxe2x80x94signal acquisition used to implement a contactless data transmission between a transponder and a base unit to acquire a low frequency signal which was modulated onto a carrier signal by means of AM modulation, and which comprises a serial oscillating circuit (L, C, R) that is excited by transistors (T1-T4), connected as an H bridge in the form of two circuit branches (T2, T3, and T1, T4), with both circuit branches (T2, T3, and T1, T4) of the H bridge being respectively driven by a single control circuit (S1, S2), and the H bridge being connected to a voltage supply source (VB), and a resistor (R1, R2) being respectively connected in series with a circuit branch (T2, T3, and T1, T4) of the H bridge.
2. Description of the Related Technology
Contactless identification systems consist of a transponder (data carrier) and a base station (write/read unit). The communication between transponder and base unit is effected by a high frequency field (carrier signal) transmitted by the base unit using a serial oscillating circuit. As the transponder does not have its own energy supply, the necessary energy must be taken from the high frequency field by means of a receiver coil. As soon as the energy taken up by the transponder is sufficient for reliable communication to be set up, the transponder will returnxe2x80x94via the receiver coilxe2x80x94a low frequency data signal, which was modulated onto the received carrier signal by AM modulation, to the base unit. In the base unit, the signal induced in the transmission coil for regaining the low frequency data signal will be separated from the carrier signal by means of an AM demodulator, and then provided to a logic unit for further processing. As the inductive coupling between the transponder coils and the base unit is small due to the low permeability constant of air, the distance between transponder and base unit must generally not be more than just a few cm. In order to transmit sufficient energy from the base unit to the transponder through the high frequency field, the amplitude of the carrier signal on the serial oscillating circuit of the base unit must be in the region of 100V. This will cause the serial oscillating circuit to be excited with its natural frequency by means of the two circuit branches of a H bridge. Due to the low coupling, even the AM modulation gain which the transponder causes in the serial oscillating circuit of the base unit will be in the region of 1 volt only. In order to increase the range, as well as to provide for a safe transmission of data between transponder and base unit solutions must be sought that will increase system sensitivity. As this is a product sector with a very high volume of quantities, it is particularly important here that all system functions are integrated in a single circuit layout at low cost.
From the data book TEMIC Semiconductor 1997 p. 352 and p. 353, circuit layouts are known where the decoupling of the modulated carrier signal is effected between the coil and capacitor of the serial oscillating circuit. As both reference points of the serial oscillating circuit change over in between ground and supply potential in line with the frequency of the carrier, the carrier signal is overlaid by a further signal with the amplitude of the supply voltage (5V). In order to separate the data signal from the carrier, the modulated signal whose amplitude is approximately 100V will be provided to an external AM demodulator. Depending on the voltage strength of the AM demodulator, the modulated carrier signal must be adapted via an additional voltage divider. Following demodulation, the separated data signal will be provided to the control IC of the base unit for further processing.
In the case of this known solution, system sensitivity and thus the range for a safe transmission of data between transponder and base unit will be low, as the modulation gain is reduced by the voltage divider, and the carrier signal is overlaid with an interference signal which is as high as the operating voltage. In addition, with this type of decoupling, only one half (the positive amplitude) of the modulated carrier signal can be evaluated. Furthermore, due to the extreme voltage strength requirements leading to high costs, it is so far not possible to integrate the demodulator circuit into the integrated circuit (IC) of the control circuit layout.
It is the task of the invention described here to state a monolithically integratable circuit layout, which willxe2x80x94by means of an increased system sensitivityxe2x80x94increase the range at which a data exchange can be effected between transponder and base unit, and at the same time allow the AM demodulator circuit to be integrated into the exciter circuit at low cost.
According to the invention this task is solved by an integrated circuit layout for LF signal acquisition pursuant to the characteristics of patent claim 1. Favorable implementations are the subject of sub-claims.
Investigations conducted by the applicant have shown that the system sensitivity of the xe2x80x9ctransponderxe2x80x94base unitxe2x80x9d arrangement will increase significantly, if the decoupling of the modulated carrier signal for LF signal acquisition by means of an AM demodulator is effected on both circuit branches of the base unit H bridge, which excite the serial oscillating circuit. As the voltage amplitude in both circuit branches of the H bridge can change over between ground potential and operating voltage only, there is no need for the voltage divider on the input of the AM demodulator, as an input voltage strength of the AM demodulator as high as the supply voltage will be sufficient. Excluding the transmitter coil of the base unit, it is thus possible to implement all circuit components in a single IC of a low blocking and highly integrative technology. In this way, the base unit can be built up with extreme compactness and at low cost. In addition, the circuit layout according to the invention also provides for implementations in bipolar as well as CMOS technologies.